Ediacaran fauna

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As Ediacaran biota , the macrofossil companies from the time of the late Proterozoic approximately 580 to 540 million years ago, respectively.

These are footprints in shelf sediments that after the most famous location in the South Australian Ediacaran hills , were named. The living beings that left these prints did not have any mineral hard part skeletons that could be preserved from fossil fuels . The representatives of the Ediacara fauna are probably multicellular animals (Metazoa). The systematic assignment of the Ediacara creatures is still controversial today.

Life in the Ediacarium

to form

The fossils of the Ediacaran fauna form a characteristic, relatively poor fossil community. About 280 taxa have been described, about half of which later turned out to be traces, parts or stages of development of other species or even as microbial colonies or inorganic formations; so only about half as many species are recognized, hardly more than about 100 to 120. Their assignment to later living forms and thus the exact systematic classification are consistently unclear and controversial. Most of the previously proposed assignments to recent animal phyla were later questioned with good arguments. All known forms were probably living on the ground ( benthic ), most of them probably not or hardly actively mobile. In 2011, Douglas Erwin and colleagues proposed a division of the shapes into different groups that have been accepted by many scientists.

Rangeomorpha

Rangea

The Rangeomorpha were modular organisms. The whole organism often resembled a fern frond with a central “stem” (“frondomorph”). This was composed of branches, each of which looked like a scaled-down copy of the entire organism, these in turn made of smaller branches with the same structure ... The whole organism was thus built up fractally over three or four levels , with the individual elements leaf-shaped in one level stood or were turned against each other The whole organism was often more or less flat, the main branches, as with Beothukis , could be at an angle to each other. Rangea was probably organized spatially around a central stem with sixfold symmetry, so that a cone-shaped shape resulted. The branches could be free, depending on the species, or they were connected to one another by a membrane. The fronds were composed of branched tubular structures. Rangeomorpha stood immobile upright on the ocean floor in life. They were anchored in the ground by a bulbous structure. Very often only the remains of the anchoring are found in the fossil record, which have left round, sometimes somewhat structured impressions in the sediment. These prints were first misinterpreted as "medusoids", as prints of jellyfish . The different types differed in the branching pattern and in the structure of the small frond elements, which could be branched on one or both sides. A reconstruction of various species was carried out using Canadian fossils, Rangea was later reconstructed again using South African fossils. The group takes its name from the genus Rangea , which was first found in Namibia and is one of the first described Ediacaran fossils. The genera Avalofractus , Beothukis , Bradgatia , Charnia , Fractofusus , Frondophyllas , Haspidophyllas , Pectinifrons and Trepassia are also assigned to the Rangeomorpha . They were initially interpreted as sea ​​feathers or octocorals , but are now considered to be a group that has died out without leaving any descendants. They make up more than three quarters of the fossil record at some sites. Although well-preserved, detailed prints have been found, no mouth opening, remnants of an intestine, gonads or other organs have ever been observed in Rangeomorpha. The widespread interpretation of their way of life is that they were suspension eaters who, however, did not take up organisms but dissolved organic molecules from the sea water (osmotrophy).

Erniettomorpha

Pteridinium

This group (alternatively also known as Petalonamae) is similar to the Rangeomorpha in that it was also made up of tubular structures; like this it was anchored in the ocean floor. The individual modules were smooth and cylindrical in shape, the fine, fractal branching of the Rangeomorpha is missing. Tubes that were parallel to each other and connected with each other in life started from the central axis. The paleontologist Adolf Seilacher compared this construction with a quilted air mattress. The tubes could also build branched, frond-like structures, but in other species they formed simpler bulbous or sack-shaped, sometimes chain-like shapes. Presumably they were wholly or partly buried in the sediment during life. The genera Ernietta , Nasepia , Palaeoplatoda , Phyllozoon , Pteridinium , Swartpuntia , Valdainia are assigned to the Erniettomorpha . The Erniettomorpha are also mostly interpreted as osmotrophic, as no indications of any other diet have been found.

Dickinsoniomorpha

Dickinsonia costata is a common member of the Ediacara biota. The systematic position is not clearly clarified, but Dickinsonia is mostly regarded as an animal living being.

Dickinsoniomorpha were thin, flat organisms lying on the ocean floor. They reveal a front and rear end, traces indicate a capacity for movement. At first glance, the organisms appear to be built up from segments arranged in series. However, this is possibly deceptive, as there are indications that the structures to the left and right of the body axis were not arranged in pairs, but rather offset from one another. This is more similar to the structure of the Rangeomorpha's fronds. It is questionable whether the “modules” of the organisms were real segments. In Dickinsonia , one of the most famous Ediacaran fossils, the modules were larger on one end and regularly decreased in size on the other; according to the trace fossils, the thicker end was in front. In Yorgia , another Dickinsoniomorpha, there is a plate with a series of body prints lying one behind the other with the print of the trace generator at the end. Perhaps the organism grew by forming new modules behind (like an earthworm). This distinguishes it from Erniettomorpha, which probably increased in size by "sprouting" modules of the same size. The traces and prints of Dickinsoniomorpha have led to the assumption that the organisms digested mats of microbes with their entire ventral (abdominal) body surface and absorbed them through the body wall, similar to recent placozoa . There is no evidence of an internal digestive system with the mouth, intestines and anus; earlier reports about this have turned out to be misinterpretations. Andiva , Dickinsonia , Epibaion , Windermeria and Yorgia are included in the Dickinsoniomorpha .

Arboreomorpha

Charniodiscus , a representative of the Arboreomorpha

In this group other organisms organized in the manner of fern fronds are combined. Like the Rangeomorpha and Erniettomorpha, these consisted of a disc-shaped or bulbous anchorage in the ocean floor, a central stem and a crown made up of branches. They differed in the branching pattern. With the Arboreomorpha the "branches" were smooth, not further branched, tube-like structures that were often swollen and then could have pear-shaped outline. They branched off roughly at right angles and were obviously connected to one another to form a leaf-shaped structure. In contrast to modern organisms with a structure similar to that of the sea feathers, water could presumably not flow through between the "branches"; this makes a filtering diet of the arboreomorpha unlikely. The Arboreomorpha include the genera Charniodiscus ( Arborea is synonymous with this), Khatyspytia , Vaizitsinia . Dimitry Grazhdankin does not consider the group to be independent, but combines them with the Erniettomorpha to form a common group called Frondomorpha. Most other paleontologists believe that the similar shape is more due to convergence . Some researchers consider thaumaptilon from the Cambrian Burgess slate to be part of it. This would be one of the few (consistently controversial) cases in which a representative of the Ediacaran fauna would have survived into the Cambrian.

Triradialomorpha

Tribrachidium

This group, called Tribrachiomorpha, Trilobozoida or just Trilobozoa by other researchers , comprises small, disk-shaped fossils with three arm-shaped structures on the surface. These are not arranged radially but twisted in a spiral. The group includes a few species, but can be represented in the fossil communities rich in individuals. The group includes the respective monotypical (only one species comprehensive) genera Albumares , Anfesta , Pomoria , Skinnera , Tribrachidium , possibly also Triforillonia . Almost all researchers accept them as fossils of real tissue animals (Eumetazoa).

Disc-shaped fossils with superimposed arms also exist with other symmetries. With Conomedusites there are four arms, with Arkarua five. This was therefore associated with the five-fold echinoderms (Echinodermata), from which they differ, among other things, in the absence of any skeletal elements. Eoandromeda even had eight spiral arms. This fossil, physically preserved in phosphate (i.e. not just as an imprint), was found in the Ediacaric Doushantuo Formation in China. Because of the great importance that symmetries normally play in the development of the basic body plan, these disc-shaped organisms are mostly viewed as not closely related to one another. However, some researchers speculate about a possible relationship.

Kimberellomorpha

Kimberella

The Kimberellomorpha may only include the genus Kimberella with a single species, Kimberella quadrata . Erwin and colleagues also classify Solza margarita from the Onega Peninsula on the Russian White Sea here. However, the first describer , Andrey Ivantsov and colleagues who also researched Kimberella , did not assign this fossil with few characteristics at all.

Kimberella was a bilaterally symmetrical organism. Trace fossils show that Kimberella was actively mobile on the substrate surface. Depending on the conservation conditions , the outline is long-oval to almost circular. When viewed from above, the body reveals two zones, a central shield-shaped with small tubercles or folds and a ring-shaped surrounding with large lacunae . A few, particularly well-preserved specimens show a trunk- like protrusion at the front end , which is sometimes surrounded by scratch-like trace fossils. Ivantsov and colleagues interpret this as a radula , the characteristic rasp tongue of molluscs (mollusca), which, however , would only have had two teeth in Kimberella . They interpret the relief of the central shield as possible remnants of a shell-like mineralization , possibly made of aragonite , which would have had no chance of preservation because of the embedding conditions of the fossils.

The interpretation of Kimberella as a "Grand Snail", the earliest representatives of the mollusks is accepted by most researchers. It is of particular importance since it is almost the only fossil of the Ediacaran fauna whose assignment to a recent animal strain would be halfway accepted. However, the interpretation is not undisputed. Jerzy Dzik sees the fossil as a Halkieriiden , an early relative of the famous Cambrian Wiwaxia , which he interprets as a poly-bristle , i.e. annelid worms . Almost all those who worked on it, however, agree that it must have been a more highly developed multicellular animal .

Bilaterialomorpha

Spriggina

This group got its name after its bilateral ( bilateral ) symmetry (it is alternatively called Proarticulata , especially by Russian researchers ). The organisms were roughly symmetrical along the longitudinal axis of the body. They had a recognizable, differentiated front and rear end. The body was divided into similar, serially arranged sections. Many biologists assume that these were segments ; others are skeptical because the sections often appear to be offset to the left and right of each other. The foremost part of the body is sometimes noticeably larger than all the others and looks a bit detached, almost like the head shield of an arthropod . Usually the sections get smaller towards the rear end. Most fossils have a thinner outer contour that connects the segments with one another, resulting in a closed body contour. Trace fossils prove the mobility of numerous representatives, also with the others it is assumed based on the anatomy. Erwin and colleagues consider the group to be a clade . This would mean that the matching characteristics result from a common descent, not just as convergent formations, for example as a result of the evolution of mobile organisms. As far as can be seen, all Bilaterialomorpha were ground-living (benthic). Some researchers claim to have seen legs or other differentiated body appendages on some forms, but these are highly controversial, and they were certainly not observed in any form. Erwin and colleagues assign the genera Archaeaspinus , Cyanorus , Ivovicia , Kharakhtia , Lossina , Marywadea , Onega , Paravendia , Parvancorina , Spriggina , Temnoxa and Vendia to the group. Dimitry Grazhdankin also inserts Kimberella , whose special position he thereby questions. Although rarely in the fossil record and never larger than about 10 centimeters, this group has always received special attention because it is assumed that the ancestors of the Bilateria , i.e. the "higher" animals, hide among them. Numerous researchers speculate about the affiliation of individual fossils to Bilateria taxa. Special attention has always been paid to possible ancestors of the Arthropoda , which emerged so prominently from the Cambrian onwards. Possible candidates such as Parvancorina and Praecambridium are viewed with skepticism today because their growth pattern (based on large and small individuals) does not match and there has never been any indication of molting . Since with the Cambrian explosion numerous, highly organized animal tribes appear more or less at once in the fossil record, which would have to be of a much older age even according to the method of the molecular clock , the assignment to the Bilateria as a whole is not unlikely. However, all assignments of Bilaterialomorpha to certain animal phyla are still controversial and doubtful.

"Sponges"

Sponges (Porifera) are considered to be the most anatomically primitive metazoa. Their appearance in the Ediacaran fauna is therefore to be expected. The discovery that the recent sponge Halichondria panicea can live in water that is very low in oxygen has caused a stir ; this would make it possible to survive in the pre-Cambrian oceans, which were long oxygen-poor. A fairly popular theory, based on some molecular data, holds that the sponges are paraphyletic even with regard to the other Metazoa . Accordingly, the sponges would not be the sister group of the other Metazoa, but these would be more closely related to some sponges than they are to each other. This would have a strong impact on how anatomically original or modern the Precambrian sponge fossils would be expected. Possibly these did not belong to one of the modern lines (the " crown group "), but they could be missing some of the autapomorphies of the modern Porifera. This is significant because there is still no convincing Precambrian sponge fossil with one of the most important diagnostic features of sponges, the skeletal needles ( spiculae ). From the early Cambrian (Tommotian) on, fossil sponges with calcareous skeletons existed with the extinct archaeocyathids ; these are very common in the fossil record. Older sponge fossils are usually based on the presence of pores on the surface (possible oscula ). Erwin and colleagues accept the genera Ausia , Palaeophragmodictya , Rugoconites and Thectardis as fossil sponges of the Ediacaran fauna. None of these interpretations are unproblematic. Ausia , a pore-bearing fossil with a triangular body contour, is possibly only a conservation-related variant of Ernietta (see above under Erniettomorpha); others see it as a fossil sea squirt or a halcyon annelid worm. Palaeophragmodictya , like so many Ediacaran fossils, is probably just the anchoring or retaining disk of another (unknown) organism. The other possible Porifera candidates are just as dubious.

Skeletal fossils

Fossils with hard parts ( skeleton ) did not appear with a greater number of forms until the beginning of the Cambrian. There were also a few representatives that were consistently problematic in the assignment in the late Ediacarium. However, these never occur together with the forms of the actual Ediacaran fauna. The finds are often found in a stratigraphic connection with the fossils of the Ediacaran fauna, in the layers in between, so they are undoubtedly the same age. There are various conceivable reasons for this: Either the skeletal forms lived in different habitats than the others, i.e. were spatially separated in life, or the conservation conditions ( taphonomy ) only allowed the fossilization of either one or the other fauna, but not both at the same time. In addition to some ambiguous and as yet undescribed forms, the genera Suvorovella , Majaella , Wyattia , Cloudina , Sinotubulites , Qinella , Protolagena , Chenmengella ( syn. Chenella ), Namacalathus and Namapoikia have been described, with only three representatives in Siberia, Anabarites , Cambrotubulus and Jacutiochrea of anabaritids, organisms of a controversial systematic position, of which only the chalky living tubes with threefold symmetry have been preserved. Anabaritides are the only proterozoic skeletal organisms that survived into the Cambrian (they died out a little later, in the middle Cambrian, after all). The other Ediacaric skeletal fossils also consist of tubes, with a few exceptions, so that their systematic position usually remains controversial. In contrast to the sometimes very large forms of the actual Ediacaran fauna, the skeletal forms remained quite small throughout, ranging in scale from millimeters to a few centimeters.

Cloudina

Cloudina is the oldest, most common and best-known skeleton-bearing animal of the Ediacariums, finds are almost worldwide (with the exception of Australia). Curved, tubular shells are often found, which consist of funnel-shaped skeletal elements with a rounded tip, which are stacked in large numbers; they reach 0.3 to 6.5 millimeters in diameter and up to 3.5 centimeters in length. There are usually too many of them embedded next to each other in fine-grain limestone; they are rarely phosphated or preserved as imprints in a silicate matrix. It is believed that in life the tubes were perpendicular to the surface. Since directly attached shells were rarely found, it was speculated about a way of life as growth on other organisms (e.g. macroalgae), but Cloudina occasionally even forms reef-like structures. Occasionally the tubes branch out, which is interpreted as asexual reproduction by means of sprouting . Cloudina lived in shallow seas with deposits of calcareous so-called microbialites ( stromatolites and thrombolites), calcium deposits precipitated by bacteria or cyanobacteria. Cloudina alsobecame famousfor the fact that the tubes occasionally have regular, round bores, this is the oldest indirect evidence of an organism with predatory nutrition that pierced the shells in order to prey on the organism living in them. Since Cloudina did not leave any soft tissue fossils , the systematic affiliation is still controversial today, most popular hypotheses are either a cnidarians (Cnidaria) or a tube-building annelid, similar to a feather worm (Sabellida).

Namacalathus

Namacalathus was first found in fossils of the Nama series in Namibia and later in Canada. The skeletons found consist of an open, tubular structure up to about 30 millimeters in length, on which a spherical structure with a maximum diameter of about 25 millimeters sat at one end. The chalky, partly organic walls were thin, single-walled and barely a millimeter thick. The spherical structure had a large circular opening at the end and six somewhat smaller such openings at regular intervals on the sides. The animal is reconstructed in such a way that in life the tubes stood upright on the sediment surface, with the spherical dome at the top protruding freely into the water. This peculiar body structure most likely indicates a cnidarian as a builder. In recent animals, however, the ultrastructure of the walls is most likely to be found in representatives of the Lophotrochozoa , even if no recent representative of this overstem with even a vaguely comparable morphology is known. An interpretation as a sponge has also been suggested.

Trace fossils

Cyclomedusa , the most common and widespread Ediacaran fossil (with a central bump that often looks broken, suggesting it was the anchoring disk of a larger organism)

The investigation of trace fossils (also called Ichnofossils ) provides significant additional knowledge. On the one hand, it provides evidence of the presence of other species, of which no compression or body fossils have survived. On the other hand, there are traces of animal behavior turned to stone, which provide valuable information on the way of life. Apart from the few cases in which traces and the trace-producing organism are fossilized in direct relation to one another, trace fossils are classified as so-called parataxa , in the case of traces called Ichnotaxa , because the trace producer is hypothetical at best and is often completely unknown. Traces directly related to the trace generator exist, for example, from Siberian Dickinsoniomorpha such as Yorgia ; Only these provided the proof that they were actually actively mobile organisms.

When considering the ediacaric Ichotaxa, a characteristic sequence can be identified. In the oldest deposits, which already contain imprints of the Ediacara fauna, mostly belonging to the various "fern frond" organisms (Frondomorpha), there are no indubitable trace fossils at all. Often only the round imprints of the anchoring and retaining discs of the frondomorphs are found here, the most common Ichnotaxon is called Aspidella , a somewhat more complex form is Cyclomedusa . Unambiguous traces can then be found in later Proterozoic sediments. Initially, these are simple furrows or channels in the sediment surface. In younger layers, the traces become a little more complex, for example drill holes below the surface of the sediment and tightly wound, meander-like feeding traces appear. More complex trace fossils are limited to the very last section of the Ediacarian, just before the base of the Cambrian. In the Cambrian, the complexity increased almost suddenly. Only now are traces of contributing organisms, interpreted as the first arthropods (or organisms from their root group ).

The simplest Ichnofossils are unbranched grooves a few millimeters wide. Since the impressions were filled in with the otherwise unchanged shape, sometimes from the sediment layer above and sometimes from the layer below, it can be deduced that they lay on the sediment surface. They are interpreted as feeding traces of organisms that they left behind when grazing on the microbial mats, which, according to most hypotheses, covered the ocean floor almost completely at the time. The regularly winding traces of later grazers (Ichnofossil Nereites ) do not yet appear in the Ediacarium; presumably the organisms had not yet reached the complexity required for this behavior. Similar traces are almost as old and are always filled with sediment from the same layer in which they lie, i.e. they were probably already in the interior of the sediment when they were formed. These are interpreted as the first sediment eater to eat their way through deposits that were rich in organic matter. In the rare habitats in which comparable microbial mats occur today, for example a hypersaline lagoon in Venezuela, recent colonizers of these mats leave very similar traces of feeding. The tracks can penetrate each other and thus form polygonal networks.

Immediately before the beginning of the Cambrian, trace fossils with a zigzag course are added, which are interpreted as the traces of a predator and / or scavenger. Such an Ichnofossil Treptichnus pedum is considered a key fossil for the border between the Ediacarian and the Cambrian. The trace producer is unknown, the most likely cause is a Priapwurm (strain Priapulida); of these, there are no body fossils with a corresponding age.

Alternative interpretations of the fossils

While most researchers, as shown above, see representatives of early multicellular animals (Metazoa) in the Ediacaran fossils, including probably representatives of extinct animal phyla, some scientists suggest radically different interpretations of these fossils. Accordingly, the fossils are not multicellular animals, or possibly not even animals at all.

Vendobionts

The paleontologist Adolf Seilacher saw in the Ediacara fossils representatives of a line of development that was completely different from the multicellular animals and that disappeared from the earth at the beginning of the Cambrian, probably due to competition from the Metazoa. He initially called these hypothetical organisms "Vendozoa", but later changed the name to Vendobionta himself . In later works he and his colleagues recognize the presence of real Metazoa in the Ediacarium. Most of the fossils, especially the Rangeomorpha, Erniettomorpha, Dickinsoniomorpha and Arboreomorpha of the above list, however, were not Metazoa. Rather, they were single-cell organisms that had grown to gigantic sizes. In support of the hypothesis, reference is made to the growth patterns unknown in Metazoa, for example the fractal growth of the Rangeomorpha, and various very large, recent unicellulars, especially skeletal foraminifera and root pods of the Xenophyophorea class, which live exclusively in the deep sea . The vendobiont hypothesis is still held by some researchers to this day, but is a minority opinion in science.

Mushrooms and lichens

According to some researchers, the Ediacara biota were predominantly fungi or lichens. In particular, the North American geologist and expert on paleo soils Gregory J. Retallack has defended his hypothesis in numerous articles that it must have been lichen . His reasoning is largely based on the fact that he accuses the other researchers of misunderstanding the deposit conditions of the fossil-bearing layers. It was not marine sediments, but fossil soils that were created in the coastal area, or even on land. Retallack's hypothesis has, apart from its inventor, found no acceptance in science.

Fossil communities

Exact biostratigraphic analysis of the fossil-bearing strata of the Ediacarian, in connection with the presence-absence data of genera and species, led to the differentiation of three fossil communities of the Ediacaran fauna: the Avalon, Nama and White Sea communities. These are named after the regions where they are particularly prominent: the Avalon Peninsula on Newfoundland , Canada, the Nama layers in Namibia, South Africa and the rock layers on the White Sea in Russia. The interpretation of these communities proved difficult. Alternatively, interpretations are based on a temporal sequence, different facies , sedimentation conditions or partial habitats or biogeographical regions with different fauna. Although the chronological sequence is striking and suggests a biostratigraphic interpretation, an interpretation as a specific fauna community of different habitats seems to best explain the data. It is speculated that when the biota of the Ediacaran fauna was displaced from deep-sea and shallow-sea sediments, they still had a refuge as far as the base of the Cambrian in fast-flowing channels that housed the Nama community.

Avalon community

The Avalon community is assigned, in addition to the eponymous deposits of Newfoundland, the finds from Charnwood Forest , England and from the Canadian Wernecke Mountains. It is characterized by a particular variety of fixed, fern-like (frondomorphic) shapes, while other elements are extremely rare. While Newfoundland and England are now separated by the Atlantic Ocean, the land masses in the Ediacarium were closely adjacent. The finds of the Canadian Wernecke Mountains are quite far apart. All sites were in relatively cool areas near the poles. The fossil-bearing rocks are interpreted as deposits from deep ocean layers; they lay below the storm wave base in the lightless aphotic zone , possibly 1.5 kilometers under water. The Avalon Community contains the oldest known Ediacaran fossils, ranging in ages from 579 million years (Newfoundland) to 559 million years (Charnwood Forest).

White Sea Community

The White Sea community is the most diverse fossil community of the Ediacaran fauna, it comprises about three times as many genera as the Avalon community. In addition to the sites on the White Sea, Ukraine and the Norwegian Finnmark , the classic sites in the Australian Flinders Range and some of the Canadian sites are also included. Typical are moving forms living on the sea floor, which are assigned to the Bilaterialomorpha and Dickinsonoimorpha. Also the "primal snail" Kimberella and almost all Triradialomorpha like Tribrachidium belong here. The places where the community was found in the Ediacarium were far apart from one another in the northern and southern hemispheres, mostly in moderate (temperate) latitudes. The fossil-bearing strata were mainly deposited in shallow water near the coast, possibly in the prodelta area of ​​river mouths. Most of the finds come from coarse-grained siliciclastic rocks. With Olenek in Siberia and the finds in the Yangtze Gorge in southern China, carbonate rocks are also part of it. Most of the fossil sites with this community are middle-aged, with most being around 555 to 550 million years old.

Nama community

(see also → Nama foreland basin )

In addition to the South African sites, the Nama community includes the finds from southern China, British Columbia (Canada) and the Mojave Desert (USA). Typical taxa are the fern-like (frondomorphic) Swartpuntia , Ernietta and the skeleton-bearing Cloudina and Namacalathus . Tubular fossils are conspicuously common, suspected of being an infaunal (buried in the sediment) way of life. In their reconstructed location in the Ediacarium, the finds of the Nama community are widely separated from one another, but strung in a striking way on a narrow band at the equator. Many of the finds come from carbonate rocks. Most of the rocks with fossils from this community are interpreted as being deposited in relatively deep water on continental slopes in moving water. The Nama community comprises the most recent Ediacaran fossils, from approximately 548 million years ago to the base of the Cambrian era 543 million years ago.

Temporal extension

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Charnia
Scale axis: millions of years ago.

First appearance

The exact time of the appearance of the Ediacaran fauna as a macrofossil community has not yet been established. This was the main reason why the beginning of the Ediacarium as a stratigraphic unit was not defined by the appearance of fossils (biostratigraphy), but by a climatic event, the end of the Marino Ice Age (also: Varanger Ice Age), about 635 million years ago has been. This ice age was a dramatic event, probably the last " Snowball Earth " episode in which more or less the entire globe, up to the equator, was covered with ice. Their decisive significance for life is shown, for example, in the carbon isotope signature. When it comes to carbon dioxide uptake, autotrophic organisms prefer the isotope 12 C to 13 C (roughly in the ratio 100 to 98), so that 12 C is enriched in the biomass. During the Marino Ice Age, the isotope ratio briefly shifted very strongly in the sediments. This is indirect evidence that at times the rate of photosynthesis on earth as a whole fell sharply. Due to the ice cover, the weathering of continental rocks decreased massively, which means that little carbon dioxide was deposited in weathering products. Due to the increasing carbon dioxide content, the Ice Age has probably more or less ended itself. A thin layer of precipitated limestone or dolomite (so-called cap carbonates ), which can be seen in the sediments worldwide, lies above the glacial sediments .

Although there are unclear findings from Canada and Kazakhstan of round, "medusoid" imprints in pre-Marino sediments, which could possibly be imprints of holding disks of frondomorphic Ediacaran organisms, typical Ediacaran fossil communities appear with some delay, rather in the second half of the Ediacarium. Other fossils, such as phosphorized cells and cell aggregates (possible embryos) in the Chinese Doushantuo formation, show that life had already become more complex. The Acritarcha , microfossils of unknown systematic classification, but probably at least partially permanent stages of protozoa, became larger and more complex almost immediately after the Marino Ice Age. The age of the oldest representatives of the Ediacaran fauna has also been corrected upwards several times, but is still around 50 million years from the end of the Marino Ice Age. Mistaken Point on Newfoundland, about 565 plus / minus 5 million years old, was considered the oldest community of more complex fossils. Finds of representatives of the genus Charnia from the Newfoundland Drook Formation are much older and reach about 579 million years old, according to more recent dates in 2016 only 571 million years old. Charnia is thus the oldest, undoubtedly transmitted fossil of the Ediacara fauna; these oldest finds already reach a body length of up to 1.85 meters. With occurrence up to the base of the Cambrian, Charnia is also one of the youngest surviving representatives of the fauna and apparently remained morphologically almost unchanged for more than 30 million years.

The end of the Ediacaran fauna

Typical fossils of the Ediacaran fauna are no longer represented in the fossil record of the Cambrian, apart from a few cases of doubt. There are few reports of typical Ediacaric forms in Cambrian sediments in Australia and the USA; these are small, poorly preserved and also come from early Cambrian deposits. More recently, fossil communities have been discovered in southern China and Kazakhstan in which ediacaric mineralized fossils such as Cloudina coexisted with representatives of a Cambrian small-shelly fauna such as Protoconodontina . Typically, the representatives of the Ediacaran fauna also disappear in the areas in which continuous, almost undisturbed sediment sequences across the Ediacarian - Cambrian border are exposed. Most of the Ediacaric representatives seem to disappear without a trace, without leaving any descendants. The reasons for the change of fauna have long been speculated, with very different hypotheses being put forward. One hypothesis assumes that the Ediacaran fauna did not suddenly become extinct, only that the conditions for their fossilization were no longer given. Imprints of soft-skinned macrofossils in coarse-grained sediment, which are typical of the Ediacarian, are unknown from later epochs. A special structure of the ocean floor with relatively stable microbial mats was probably required for its maintenance, which later no longer existed in this form. But since there are Cambrian fossil sites with numerous fossils with soft tissue preservation, most researchers believe in real extinction. Another hypothesis assumes a real mass extinction on the Ediacarian – Cambrian border. According to this, it was only the disappearance of the Ediacaran fauna that would have set the stage for the fauna of the Cambrian (which was previously rare and inconspicuous and so hardly ever fossilized). An important argument for this is that a shift in the ratio of the carbon isotopes 12 C compared to 13 C can be observed around the world, which could be a sign of a massive disruption of the biological carbon cycle and an indication of anoxic conditions in the ocean floor. Against this, however, speaks that there were comparable episodes during the Ediacarium without drastic consequences; Moreover, most of the representatives did not suddenly die out on the border with the Cambrian, but the only surviving Nama community at that time was already clearly impoverished in terms of forms and species. Further hypotheses assume that the Cambrian fauna itself displaced the Ediacaran fauna. Either effective predators appeared for the first time , simply eating up the species that were defenseless against them, or the Ediacara biota would not have been able to cope with the superior competition of modern fauna.

It seems most likely today that the Ediacaran fauna could not cope with the changes in habitats in the Cambrian that were brought about by a modern fauna. Important innovations in the Cambrian included the evolution of effectively burrowing and burrowing forms that plowed the ocean floor and destroyed the stable microbial mats that previously covered the bottom. With that, the sessile Frondomorphs literally lost their grip. In addition, the first free swimming (pelagic) animals did not appear until the Cambrian. The food web of the oceans could have been turned upside down by tiny, phytophagous zooplankton. The representatives of the Ediacara fauna are mainly fed by dissolved organic substances (osmotrophy) or by breaking down the microbial mats. Both were probably no longer possible.

Paleoecology

The Neoproterozoic is characterized by the transition of the deep ocean basins from the oxygen-free (anoxic) state rich in sulfides ( euxinic ) to deeply oxygen-saturated water, as is characteristic of today's oceans. The Ediacarium was characterized by pronounced fluctuations in the oxygen content of the sea, which are closely related to the icing cycles. The trend towards steadily increasing oxygen levels was interrupted during the Ice Ages, when the deep water was probably rich in bivalent (reduced) iron, but poor in sulfide. In Newfoundland, the first fossils of the Ediacaran fauna in the Drook Formation immediately follow the sediments of the Gaskiers Ice Age . These are the first sediments in which there is no longer any evidence of reducing conditions on the ocean floor. The assumption is that this connection is not accidental. The oxygenation of the deep sea probably gave the starting signal for the Ediacaran fauna, which therefore could not have emerged at an earlier point in time. (However, other data indicate that at least the deeper ocean basins may have remained poor in oxygen until the end of the Proterozoic.) At least small multicellular organisms themselves must have emerged earlier as precursors in this case, possibly as a defense strategy against newly evolved predatory ones Protozoa . However, fossils of these (hypothetical) multicellular organisms are controversial. The development of the Ediacaran fauna was not an isolated event. In the Ediacarium, for example, the diversity of forms and species of the acritarcha and the more complex, multicellular algae took a similar course.

Although the biodiversity of the communities of the Ediacaran fauna could at least have reached the level of later biocenoses , their living conditions were completely different and the ecological specialization of the different species was much less than later in the Phanerozoic . The organisms presumably lived primarily on microbial mats that covered the ocean floor extensively and that they grazed or in which they could create shallow passages. The immobile, upright frondomorphs with no evidence of a mouth or intestine, or even pores and openings down to a tenth of a millimeter, were probably osmotrophs that absorbed dissolved organic matter directly from the water. (However, there are also alternative interpretations: Thomas Cavalier-Smith suspects a filtering diet in which the cells serving for nutrition (homologous to the choanocytes of recent sponges) would have been on the outside instead of the inside of the organism.) There is neither Evidence of predators (although simple predatory organisms such as Cnidaria need not necessarily leave bite marks) nor of filter feeders. The ocean floor was probably structured completely differently than later. Since there were no deep-burrowing animals, the decomposition of organic matter took place almost entirely on the substrate surface, and mats and coatings made of autotrophic cyanobacteria were probably added. These microbial mats are, where they still exist today, mechanically stable, they can be pulled off and rolled up. The ocean floor below was believed to be anoxic. Since there was no filtering zooplankton, the phytoplankton probably sank extremely slowly. Due to the leaching of the dead cells, the ocean was probably enriched with dissolved organic molecules compared to today - food basis for the osmotrophs. So the habitat of the Ediacaran fauna may have been just as unfamiliar and exotic to us as its inhabitants.

The importance of the Ediacaran fauna

Just a few decades ago, it was assumed that multicellular organisms only existed since the Cambrian , about 542 million years ago, since hard parts have only been widely developed since then (“ Cambrian explosion ”). This view was put into perspective with the discovery of the much older Ediacara biota . The interpretation of the Ediacara fossils then fluctuated for a long time between the classification of almost all fossils in recent animal phyla or the classification in a completely alien realm of the "Vendobionts". More recent findings and findings led to a more differentiated view. This began with the realization that the "medusoids", the most common Ediacaran fossils, could not have been fossilized jellyfish , as was assumed until the 1990s . The similarity of the Frondomorphs to the recent sea ​​feathers , which was often cited in the past , was also unmasked as only external similarity (the oldest unambiguous fossils of sea feathers come from the Jurassic , actually feather-like forms are even more recent.) Today the different fossils are mostly believed to belong to different lineages and not necessarily all considered as similar or related to one another. The discovery of new types of deposits with fossils from the Ediacarium in limestone or new fossils such as the enigmatic Coronacollina acula with skeletal elements already in the Ediacarium show that many previous interpretations were too narrow. Despite detailed, new reconstructions based on excellently preserved fossil material, most of the fossil representatives have remained a mystery to this day. Organisms such as the fern-frond-like Frondomorphs or Dickinsonia with lengths up to the meter range and a thickness of only six millimeters defy a satisfactory interpretation to this day. Proposals to drop the term “Ediacaran biota” as misleading are probably premature. What the Ediacara biota really were, how they lived, why they became extinct and what exactly happened during the transition into the Cambrian are scientifically open questions to this day.

Research history

The paleontologists and evolutionary biologists of the 19th century were unaware of any Precambrian fossils. Charles Darwin wrote in his major work On the Origin of Species in 1859:

“Consequently, if my theory be true, it is indisputable that before the lowest Silurian stratum was deposited, long periods elapsed, as long, or probably far longer than, the whole interval from the Silurian age to the present day; and that during these vast, yet quite unknown, periods of time, the world swarmed with living creatures. "

“Therefore, if my theory is true, it is indisputable that long periods of time, as long, or possibly longer than the entire, must have passed before the oldest Silurian layers [these“ Silurian ”deposits were later dated to the Cambrian] Period from the Silurian until today; and during all of these huge, hitherto almost unknown epochs, the world was filled with a swarm of living creatures. "

Geologists like Charles Lyell assumed that all older deposits had been destroyed by erosion or metamorphosis, Charles Walcott blamed their absence on the fact that they had lived in a (purely hypothetical) "Lipali Ocean", whose deposits had not been preserved. By problematic fossils such Eozoon and Cryptozoon quenched (both later than misinterpreted unmasked) ventured long time no more paleontologist to postulate Precambrian fossils. Accordingly, the finds of mysterious organisms in Newfoundland in 1868, in Namibia in 1933 and in the Ediacara Hills north of Adelaide, Australia in 1949 (by Reginald Claude Sprigg ) were initially dated to the Cambrian. This did not change until 1957 when the student Roger Mason discovered an unknown fossil in Charnwood Forest, England, which was later named Charnia masoni in his honor (it was not known until much later that the first fossils were discovered here in 1848, but not published had been). The English geologist Trevor D. Ford was able to prove in 1958 that the strata in which Charnia was found were older than the Cambrian deposits. While Ford assumed fossilized algae, the Australian paleontologist Martin Glaessner assumed that it was the remains of a previously unimaginably ancient fauna. Since Glaessner not only published in academic journals, but also presented his results in popular publications, the Australian finds became far better known than any other. The expression Ediacara fauna first used by him caught on and is still used today.

distribution

Ediacara biota have been found on all continents (except Antarctica ) worldwide . More than forty sites have now been described worldwide. The most important fossil deposits are the Flinders Range in Australia (with the eponymous Ediacara Hills), the southeast of the island of Newfoundland (Canada), the coastal region of the White Sea (Russia) and the Kalahari craton in Namibia. New sites are still being discovered, for example in Brazil and China in 2014.

etymology

The name Ediacara (or Idyacra) comes from the native people of Australia. The name has been handed down in writing since around 1859, when the first European settlers settled in the area. Its etymology should refer to a place where or near where water is present or was present in a bygone era. It was introduced to the fossil community by the paleontologist Martin Glaessner. For decades, numerous, especially Russian, paleontologists preferred the name “Vendic” fossils (after the Vendian ) instead .

literature

  • Paul Selden and John Nudds: Window on Evolution - Famous Fossil Sites in the World (translated by Jens Seeling). Elsevier Spektrum Akademischer Verlag, Munich 2007, ISBN 978-3-8274-1771-8

Web links

Commons : Category: Ediacaran Fossils  - Collection of images, videos and audio files

Individual evidence

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